Date: Jan 19, 2009 7:50 PM
Subject: Re: Circular functions

On Jan 19, 11:44 pm, Albert <> wrote:
> On Jan 20, 10:25 am, Matt <> wrote:

> > You've seen that cos(angle) = x/r
> <snip>
> > If the angle is 90 degrees then what is the
> > corresponding value of x?

> Now I see: x is zero and therefore cos90 = 0.


Possibly I'm just repeating what's already been said, but imagine a
ray of length 1 that has one end fixed at the origin, and the other
end sweeping out a circle of radius 1. The x-coordinate of the free
end is cos(A), and the y-coordinate is sin(A), where A is the angle
the ray makes with the positive x-axis.

Anticlockwise rotation from the positive x-axis means a positive
angle; clockwise rotation means a negative angle. Once you've gone
around a full circle (2*pi radians, 360 degrees) you're back where you
started, so sine and cosine are periodic with period 2*pi.